The present invention relates to a high-voltage pulse generating circuit for use in discharge-excited lasers such as copper vapor lasers, excimer lasers, etc. and accelerators such as linear induction accelerators, and more particularly to a high-voltage pulse generating circuit comprising a magnetic pulse compression circuit.
Discharge-excited lasers such as copper vapor lasers, excimer lasers, etc. are expected to be used for chemical reaction processes such as uranium enrichment, lithography, CDV, etc.
Such discharge-excited lasers are required to have high output, high pulse-repetition rate, high reliabilty and long service life. For achieving these requirements, a high-voltage pulse generating circuit as shown in FIG. 24 is used. This high-voltage pulse generating circuit 24 comprises a variable high-voltage dc power supply 1, a resistor 2 for charging a main capacitor 5, a thyratron 3, an inductor 4, a capacitor 6, a peaking capacitor 8, main laser discharge electrodes 9, a main saturable reactor 10, and an inductor 81 for charging the main capacitor 5.
Explanation will be made referring to FIGS. 24, 25 and 28 on the operation of this circuit when parameters of the constituent elements are optimized such that an energy transmission efficiency from the main capacitor 5 to the peaking capacitor 8 is maximum.
In the turn-off period of the thyratron 3, the main saturable reactor 10 is reset from a point "e.sub.8 " to -Br via a point "a.sub.8 " in FIG. 25, by a magnetizing force generated by current for charging the main capacitor 5 which flows through a course from a plus terminal of the dc power supply 1 to the resistor 2, the inductor 4, the main capacitor 5, the winding 11 of the main saturable reactor 10, the inductor 81 and a minus terminal of the dc power supply 1. In this circuit, a circuit for charging the main capacitor 5 serves also as a reset circuit for the main saturable reactor 10.
Next, when the thyratron 3 is turned on at t=0 in FIG. 28, terminal voltage v.sub.6 of the capacitor 6 increases, as shown in FIG. 28 (a), in the polarity shown in FIG. 24, by discharge current i.sub.1 shown in FIG. 28 (b) which flows through a course from a plus terminal of the main capacitor 5, to the inductor 4, the thyratron 3, the capacitor 6 and a minus terminal of the main capacitor 5. During this period, the magnetic flux density of the main saturable reactor 10 changes from -Br toward a point "b.sub.8 " in FIG. 25. At this time, since the main saturable reactor 10 has an extremely large inductance L.sub.10, current i.sub.2 flowing through a course from the capacitor 6 to the capacitor 8, the winding 11 of the main saturable reactor 10 and the capacitor 6 is much smaller than the current i.sub.1 as shown in FIG. 28 (e). Thus, the main saturable reactor 10 is in a turn-off state equivalently. Therefore, as shown in FIG. 28 (c), the main saturable reactor 10 blocks the voltage in a polarity shown in FIG. 24.
When the current i.sub.1 becomes zero at t=.tau..sub.1, the magnetic flux density of the main saturable reactor 10 reaches a point "b.sub.8 " in FIG. 25, so that a magnetic core of the main saturable reactor 10 is saturated. At this time, the main saturable reactor 10 has inductance L.sub.10 (sat.) sufficiently smaller than the inductance L.sub.4 of the inductor 4, so that most of charges stored in the capacitor 6 flow as current i.sub.2 in the direction shown in FIG. 24. As shown in FIG. 28 (e), i.sub.2 drastically increases, so that the magnetic flux density of the main saturable reactor 10 changes from a point "b.sub.8 " to a point "Br" via a point "c.sub.8 " in FIG. 25. Accordingly, energy stored in the capacitor 6 is mostly transmitted to the peaking capacitor 8 as shown in FIG. 28 (discharge-excited laser).
Incidentally, a period from a time at which the thyratron 3 is turned on, to a time at which the current i.sub.2 becomes zero is called a "gate period." Assuming that each element suffers from no loss, ##EQU1## E: Input dc power supply voltage (V). N.sub.11 : Number of winding of coil 11 of main saturable reactor 10.
Ae: Effective cross section (m.sup.2) of main saturable reactor 10. PA1 .DELTA.B.sub.m : Operating magnetic flux density (T) of main saturable reactor 10. PA1 Bs: Saturation magnetic flux density (T) of main saturable reactor 10. PA1 Br: Residual magnetic flux density (T) of main saturable reactor 10 in the saturation region. PA1 L.sub.4 : Inductance (H) of inductor 4. PA1 L.sub.10(sat) : Inductance (H) of main saturable reactor 10. PA1 C.sub.5 : Capacitance (F) of main capacitor 5. PA1 C.sub.6 : Capacitance (F) of capacitor 6. PA1 C.sub.8 : Capacitance (F) of peaking capacitor 8. PA1 H.sub.LM : Wave height (A/m) of gate magnetizing force of main saturable reactor 10. PA1 I.sub.2m : Wave height (A) of i.sub.2. PA1 le: Mean magnetic path length (m) of main saturable reactor 10.
As soon as all the energy of the capacitor 6 is transmitted to the peaking capacitor 8, the main laser discharge electrodes 9 are broken down at a time of .tau..sub.1 +.tau..sub.2 shown in FIG. 28, so that the energy of the peaking capacitor 8 is consumed in a laser gas. At this time, although most energy accumulated in the peaking capacitor 8 is consumed in a laser gas via the main laser discharge electrodes 9, a part of the energy is used to reset the main saturable reactor 10. By this energy, the magnetic flux density of the main saturable reactor 10 changes from a point "Br" to a point "e.sub.8 " via a point "d.sub.8 " in FIG. 25.
The above operation is usually repeated at a predetermined pulse-repetition rate.
When the current discharged from the main capacitor 5 is smaller than current necessary for generating a full-reset magnetizing force Hr (determined by operation conditions) of the magnetic core of the main saturable reactor 10, a reset circuit 85 may be added, as shown in FIG. 26, to reset the main saturable reactor 10. The reset circuit 85 is connected to terminals 83, 84 of a reset winding 82 for the main saturable reactor 10, to reset the main saturable reactor 10 to an opposite magnetization direction as shown by the dot shown in FIG. 26. The details of the reset circuit 85 are described in Japanese Patent Laid-Open No. 63-171172, etc. FIG. 27 shows an example of the reset circuit 85 having output terminals 83, 84. The reset circuit 85 comprises an inductor 86 for blocking a high-voltage surge induced in the reset winding 82 of the main saturable reactor 10 during the gate period, a resistor 87, a varistor 88 and a dc power supply 89.
In the above conventional circuit, there is one magnetic pulse compression circuit comprising a saturable reactor, but some high-voltage pulse generating circuits comprise semiconductor elements such as thyristors instead of thyratrons as switching elements, and multistage magnetic pulse compression circuits consisting of a plurality of magnetic pulse compression circuits each comprising a saturable reactor. Also, in the case of accelerators such as linear induction accelerators, high-voltage pulse generating circuits comprising multistage magnetic pulse compression circuits are mostly used because large output is required.
Incidentally, the principle of a magnetic pulse compression circuit is described in "The Use of Saturable Reactors As Discharge Devices for Pulse Generators," W. S. Melville, Proceedings of Institute of Electrical Engineers, (London) Vol. 98, Part 3, No. 53, pp. 185-207 (1951); the application of such circuit to discharge-excited lasers is described in "Electrical Excitation of an XeCl Laser Using Magnetic Pulse Compression," I. Smilanski, S. R. Byron and T. R. Burkes, Appl. Phys. Lett. 40 (7), pp. 547-548 (1982); the magnetic pulse compression circuit using semiconductor elements is described in U.S. Pat. No. 4,549,091, and "An Efficient Laser Pulser Using Ferrite Magnetic Switches," H. J. Baker, P. A. Ellsmore and E. C. Sille, J. Phys. E. Sci. Instrument 21 (1988), pp. 218-224.
Also, in accelerators such as linear induction accelerators for free electron lasers, etc., high-voltage pulse generating circuits having the same system as described above may be used. The details are described, for instance, in D. Birx, E. Cook, S. Hawkins, S. Poor, L. Reginato, J. Schmidt and M. Smith: "The Application of Magnetic Switches as Pulse Sources for Induction Linacs," IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, pp. 2763-2768 (1983), and U.S. Pat. No. 4,730,166.
In discharge-excited lasers, the stabilization of laser output and the reduction of jitter are required. For instance, in excimer lasers for lithography, it is necessary to stably supply a laser output of about 100 mJ per one pulse for a period of 10.sup.8 shots or more in a pulse-repetition rate of about 500 Hz. However, since a laser gas is deteriorated by repeated operation, it is necessary to gradually increase an energy to be supplied to the laser gas, in order to satisfy the above output requirements. For this purpose, in the conventional circuit shown in FIG. 24, the input dc power supply voltage is gradually increased. In the circuit shown in FIG. 24, since the operating magnetic flux density (.DELTA.B.sub.m expressed by the formula (4)) of the main saturable reactor 10 in a gate period is constant, voltage and current at main elements in the circuit have waveforms shown in FIG. 29, when the input dc power supply voltage is lower than an optimum value at which the energy transmission efficiency from the main capacitor 5 to the peaking capacitor 8 is maximum. On the other hand, when the input dc power supply voltage is higher than the above optimum value, the voltage and current waveforms become as shown in FIG. 30. In both cases, the energy transmission efficiency from the main capacitor 5 to the peaking capacitor 8 decreases, and after-current of the current i.sub.1 flowing between main electrodes of the thyratron 3 increases, causing inverse current to flow. As a result, the loss of the thyratron 3 increases. Further, since a percentage of energy which does not contribute to the laser oscillation increases in the laser gas, the service life of the laser gas decreases. Therefore, the number of shots by which a constant laser output can be obtained is limited to about 10.sup.6 or less.
In the copper vapor lasers used in a uranium enrichment process, stable, continuous operation is required at a pulse-repetition rate of about 5 kHz or more and at a laser output of about 100 W with a jitter of .+-.3 nanoseconds or less for about 1000 hours or more. Since such lasers are operated at a pulse-repetition rate about one order higher than that of the excimer laser, it is strongly desired to use a high-voltage pulse generating circuit comprising a multistage magnetic pulse compression circuit and semiconductor elements such as thyristors instead of thyratrons as switching elements. However, in the conventional high-voltage pulse generating circuit utilizing a multistage magnetic pulse compression circuit, to optimize the energy transmission efficiency from the main capacitor to the peaking capacitor at a final stage, a pulse width of current flowing after the saturation of the saturable reactor should be adjusted. For this purpose, an inductor is inserted in series to a saturable reactor in each magnetic pulse compression circuit. This is because the operating magnetic flux density of the saturable reactor constituting each magnetic pulse compression circuit in a gate period is constant as .DELTA.B.sub.m in the above formula (4). In addition, the above procedure should be utilized in the adjustment of the magnetic pulse compression circuit in the synchronous operation of a plurality of high-voltage pulse generating circuits, so that it is extremely difficult to use such system in commercial plants needing the synchronous operation of a plurality of high-voltage pulse generating circuits.
In free electron lasers or linear induction accelerators used for plasma heating of nuclear fusion plants, a kind of transformer for accelerating electron beams, which is called "accelerator cell," should be supplied with rectangular pulses having a voltage wave height of several hundreds of kV, a current wave height of several tens of kA and a pulse width of about 100 nanoseconds, with jitter within several nanoseconds at a pulse-repetition rate of several kHz or more in a burst mode for as long a period of time as possible. In the high-voltage pulse generating circuit in these applications, a multistage magnetic pulse compression circuit comprising thyratrons as switching elements in parallel is used. In this high-voltage pulse generating circuit, there is a problem that the energy transmission efficiency decreases as the operation time passes, since the operating magnetic flux density of the magnetic core of the saturable reactor in a gate period decreases by repeated operation because of temperature rise caused by the loss of the saturable reactor.